Nukes are destructive, and yes, they blow up everything in range.
One easy way to kill off humanity.
But, say, some person comes along and looks around. He sees the imperfections everywhere and believes that if he were in control, he could change everything. Completely eradicate poverty, debt, remove the idea of money and military...
The guy knows that there is no way to eliminate everything mentioned above by advocacy alone, nor by force, so a discreet mass extinction of humanity is the only option. But Mr. Person and his wife has to survive so that they start life over and rebuild humanity.
How would Person do this?
- Mr. Person wants to start life all over again. He is willing to cause the deaths of people and animals to achieve this.
- Person does not want the world contaminated. This basically means no chemical, biological, or nuclear weapons. Nukes strictly prohibited.
- He needs to ensure that there are minimal survivors.
- Mr. Person and his wife have to survive, so that life does not become extinct, so they can rebuild humanity.
- Apparently, Person has a googolplex dollars, which is a very large number, so our subject has anything at his disposal.
I know this question exists, and it is similar, but in that question, the goal of the question was to comepletely eliminate humanity with only one person alive. The virus proposed in that question would have worked just fine. With this question, the environment still has to be habitable so that life can continue living above ground. Much of the Earth should still be suitable to life without the need of any life sustaining devices (air tanks, gas masks, etc.) that would have been required in the other question's scenario, as Mr. Person intends to restart humanity with the same territorial borders, but a more peaceful foundation. That's why the question above is hardly related to mine.
Countering answers on that question, you can't mind control people in unknown's scenario, for the asker of that question has told people to keep the answers within reasonable boundries. Turning people into memetics is kind of out of the reach of humanity at that time... Also, as proposed in another answer, killing off many people with radiation is bound to leave traces, and here, Mr. Person does not want to recreate humanity on a radioactive foundation.